1. Field of the Invention
This invention relates to a focus detecting apparatus for use in optical instruments such as cameras.
2. Description of the Prior Art
In optical instruments such as cameras, focus detecting apparatuses are well known in which the amount of relative deviation of a plurality of object images formed on the basis of lights passed through different pupil areas of an objective lens is detected by a sensor to thereby detect the focus state of the objective lens.
For example, an apparatus in which a fly-eye-lens is disposed in the predetermined imaging plane (a plane equivalent to the film surface) of the photo-taking lens of a camera and a sensor array is disposed rearwardly of the fly-eye-lens, whereby the amount of deviation of images corresponding to the focus state of the photo-taking lens is detected is disclosed in U.S. Pat. No. 4,185,191 (issued Jan. 22, 1980). Also, so-called secondary imaging type apparatuses in which a plurality of imaging lenses are juxtaposed rearwardly of the predetermined imaging plane of the photo-taking lens, whereby a plurality of object images are directed to a sensor array to thereby detect the amount of deviation of the images are disclosed in Japanese Laid-open patent application No. 118019/1980 (laid open on Sept. 10, 1980) and Japanese Laid-open patent application No. 155331/1980 (laid open on Dec. 3, 1980). The apparatus of this type has a more or less greater full length than the aforementioned apparatus, but it has a merit that it does not require any special optical system such as a fly-eye-lens.
The principle of the secondary imaging type focus detecting apparatus will hereinafter be described briefly by reference to FIG. 1 of the accompanying drawings. A field lens 2 having the same optic axis as that of a photo-taking lens 1 whose focus is to be adjusted is disposed in the predetermined imaging plane of the photo-taking lens 1 and two secondary imaging lenses 3a and 3b are parallel-disposed rearwardly of the field lens 2, and light-receiving sensor arrays 4a and 4b each comprising a plurality of photoelectric conversion elements are further disposed rearwardly of the secondary imaging lenses. Reference characters 5a and 5b designate stops provided near the secondary imaging lenses 3a and 3b. The field lens 2 substantially images the exit pupil of the photo-taking lens 1 on the pupil planes of the two secondary imaging lenses 3a and 3b. As a result, light fluxes entering the secondary imaging lenses 3a and 3b emerge from the regions of equal area on the exit pupil of the photo-taking lens 1 which correspond to the secondary imaging lenses 3a and 3b and which do not overlap each other. When the primary image O' of an object O formed near the field lens 2 by the photo-taking lens 1 is re-imaged as secondary images O" on the light-receiving surfaces of the sensor arrays 4a and 4b by the secondary imaging lenses 3a and 3b, the re-imaged two secondary images O" vary their positions on the basis of the difference between the positions in the direction of the optic axis at which the primary image O' is formed.
FIGS. 2A, 2B and 2C of the accompanying drawings illustrate the manner in which such phenomenon occurs. The two secondary images O" formed on the light-receiving surfaces of the sensor arrays 4a and 4b in the near-focus state and the far-focus state as shown in FIGS. 2B and 2C with the in-focus state of FIG. 2A as the center move in the opposite direction on the light-receiving surfaces of the sensor arrays 4a and 4b. If the then distributions of quantity of light of the secondary images O" are photoelectrically converted into electrical signals by the sensor arrays 4a and 4b and these signals are processed by an operating circuit to thereby detect the amount of relative positional deviation of the two secondary images O", it will become possible to discriminate the focus state of the photo-taking lens 1.
As a method of processing the photoelectrically converted signals, there is, for example, a method which will hereinafter be described. When the photoelectrically converted signals of the sensor arrays 4a and 4b each having N photoelectric conversion elements are a(i) and b(i) (i=1-N) respectively when the secondary images O" are photoelectrically converted by the sensor arrays 4a and 4b, in the previously described example, the operation of ##EQU1## is effected for a suitable constant K by an analog circuit or a digital circuit. It is to be understood that in equations (1) and (2), min [x, y] represents the smaller one of the two real numbers x and y and max [x, y] represents the greater one of the two real numbers x and y. Also, as regards K, k=1 is usually chosen, and the direction of movement of the phototaking lens 1 to the in-focus position is indicated by the positive or the negative sign of the operated value V1 or V2.
An example of the operation of equations (1) and (2) will now be described by the use of the example of photoelectrically converted signals of FIG. 3 of the accompanying drawings. The curves A and B of FIG. 3 represent the photoelectrically converted signals a(i) and b(i) of the secondary images O" and, in this case, it is to be understood that N=14 and the signals are operation-processed from a1, b1 to a14, b14. Assuming that the signal at the point a1 is represented as A1, the signal at the point b1 is represented as B1, and so forth, the first term V11 of equation (1) is EQU V11 =min[A1, B2]+min[A3,B4]+.. +min[A13,B14] (3)
and if this is specifically applied to the example of signals of FIG. 3, EQU V11=B2+B3+B4+ . . . +A13 (4)
and likewise, with regard to the second term V12 of equation (1), ##EQU2## Accordingly, in this case, V1=V11-V12 of equation (1) apparently becomes positive. During the in-focus, V1 is 0 and therefore, if the photo-taking 1 is designed to be driven in a predetermined direction in accordance with the positive or the negative sign of V1 as shown in FIG. 4A of the accompanying drawings, the in-focus state can be approached.
Also, in the case of equation (2), the first term V21 thereof is ##EQU3## and the second term V22 of equation (2) is ##EQU4## Accordingly, V2=V21-V22 of equation (2) differs in sign from V1 in the previous case, namely, becomes negative. Likewise, V2 is 0 in the in-focus and therefore, if the photo-taking lens 1 is driven in the direction opposite to the case of V1 in accordance with the positive or the negative sign of V2 as shown in FIG. 4B of the accompanying drawings, the in-focus state can be approached.
Accordingly, if the photoelectrically converted signals obtained as described above are operated by the signal processing system of equation (1) or (2), a focus detecting apparatus can be constructed entirely equivalently on the basis of the operated values of the two equations with the exception that the signs of the operated values are opposite to each other.
However, the signal processing system by equation (1) or (2) causes a phenomenon that the accuracy of one system is reduced when the photoelectrically converted signals present a certain special pattern, and the operation effect differs greatly between equations (1) and (2). Such phenomenon will now be described by reference to FIGS. 5A, 5B and 5C of the accompanying drawings. In the specific example of the photoelectrically converted signals shown in FIG. 5A, it is to be understood that for simplicity, N=8 and signal A(a20-a27) is signal outputs A20=A21=M1, A22=M2, A23=M3 and A24=A25=A26=A27=M4 (M4&gt;M3&gt;&gt;M2&gt;M1) and signal B(b20-b27) is the signal A displaced to the right by one signal. At this time, the operated values V1 and V2 of equations (1) and (2) are: ##EQU5## Accordingly, V1 and V2 of equations (8) and (9) give operated values opposite in sign to each other. This result corresponds to the example of the operation described by reference to FIG. 3.
An attempt is now be made to find V1 and V2 in the signal pattern shown in FIG. 5B. This signal pattern is the signal pattern of FIG. 5A in which both signals A and B have been displaced to the left end by 3 bits and likewise, the signal A(a30-a37) and the signal B (b30-b37) are displaced by one signal. At this time, V1 and V2 are ##EQU6## As is apparent from equations (8) to (11), the operated values V1 and V2 in FIG. 5B are both small as compared with the case of FIG. 5A, and particularly V2 is remarkably small. That is, in the pattern as shown in FIG. 5B, the signal processing system of equation (2) is reduced in accuracy as compared with that of equation (1) and there is an undesirable possibility that in-focus is judged by mistake in spite of the photo taking lens being not in focus and that an operated value of the opposite sign is put out under the influence of noise or the like.
An attempt is now made to find V1 and V2 in the signal pattern shown in FIG. 5C. The signal pattern of FIG. 5C is the signal pattern of FIG. 5A displaced to the right end by 4 bits. At this time, V1 and V2 obtained from A(a40-a47) and B(b40-b47) are ##EQU7## and in this case, in FIG. 5C, particularly V1 is remarkably small. That is, in the signal pattern shown in FIG. 5C, the signal processing system of equation (1) is reduced in accuracy as compared with that of equation (2).
Summing up, the signal processing systems of equations (1) and (2) do not differ in accuracy from each other in the signal pattern in which there is no signal output difference at the end portion of the operation area as shown in FIG. 5A, the system of equation (2) is inferior to the system of equation (1) in the signal pattern in which the signal output becomes sharply smaller only at the end portion of the operation area as shown in FIG. 5B, and the system of equation (1) is inferior to the system of equation (2) in the signal pattern in which the signal output becomes sharply greater only at the end portion of the operation area as shown in FIG. 5C. Accordingly, if the signal processing system of equation (1) or (2) is singly used, there will be an undesirable possibility that the focus detection accuracy is reduced in the special pattern as described above.